Chemical fertilizers do not lose potency, but they absorb water from the air so they take on weight. Therefore 16 ounces camn become 18 ounces. It will still only have the strength of the original 16 ounces.
The fertilizer is fine as long as it will all dissolve. If it has formed clumps it will take longer to dissolve. Dissolving it in hot water and letting it cool is fine. If it does not all dissolve or if it forms precipitates I would replace it rather than filtering and using it.